1. Field of the Invention
This invention relates to a photo-sensor for reading information such as characters and patterns. The photosensor of this invention is especially useful when employed as image sensors for facsimile equipment, optical character recognition, etc.
2. Description of the Prior Art
Heretofore, a linear silicon photodiode array has been generally employed as the photosensitive elements of a facsimile transmitter, optical character recognition etc. Since, however, silicon is subject to limitations in the size of a producible single crystal and the processing technique, the length of the linear silicon photodiode array has a limit. At present, the length of the linear silicon photodiode array is only about 30 mm at the utmost. On the other hand, an original picture to be read has a width of, for example, 210 mm in the A4-size or format. Accordingly, in case of reading the original picture of the A4-size with the linear silicon photodiode array, the original picture is scaled down and imaged on the linear silicon photodiode array by the use of a lens system.
In case of using such a lens system, there are the disadvantages that the positioning of the lens requires much labor and that a degraded resolution of a peripheral part and an insufficient quantity of light arise.
Recently, it has been attempted to eliminate the disadvantages of such a lens system by employing optical fibers as an optical system.
An example of the attempt is described in detail in, for example, "Gazo Denshi Gakkai Shi (Bulletin of the Picture Electronics Society)," vol. 4, No. 2, pp. 54-61 (1975). Hereunder, the technique will be briefly explained.
FIG. 1 is a view showing the operating principle of the prior art. In the figure, numeral 1 designates an original picture, and numeral 2 denotes 1,280 optical fibers. One side of the optical fibers close to the original picture is in the form of a sheet, while the other side is distributed to 20 linear silicon photodiode arrays 3. The linear silicon photodiode array consists of 64 photodiodes. The photo-sensor having such a structure is meritorious over the photo-sensor employing the lens system in that the degradation of the resolution of the peripheral part and the insufficient quantity of light can be avoided, but it is not practical on account of disadvantages as listed below.
1. The optical fibers of the structure in which one side is in the form of the very thin sheet and the other side is distributed to the 20 linear silicon photodiode arrays, eventually need some extent of length. This is unfavorable for the miniaturization of the device. Moreover, there is a high possibility that the optical fibers will be broken by mechanical shocks such as vibrations.
2. Inasmuch as the optical fibers must be arrayed in perfect conformity with the pitch of the silicon photodiodes, fibers of high precision are required. The alignment takes much labor, and the position is prone to shift in the course of use.
3. In order to prevent the destruction of the linear silicon photodiode array, the optical fibers must be floated in use. This degrades the resolution drastically.
Further, the inventors have proposed a new photo-sensor as described hereunder. The photo-sensor is already the subject of a patent application in the United States.
FIGS. 2 and 3 show an example of the photo-sensor. FIG. 2 is a plan view of the example, while FIG. 3 is a side view thereof.
Referring to the figures, numeral 4 designates a bundle of optical fibers, and numeral 5 a plate of glass which can be readily bonded with the optical fiber bundle 4. Hereinafter, a plate which has a bundle of optical fibers buried in at least its inner part in this manner shall be termed "fiber substrate." Parts 6 are transparent stripe electrodes. Numeral 7 indicates a semiconductor layer, and numeral 8 an electrode made of a thin metal film. In this manner, a photodiode array which consists of the transparent stripe electrodes 6, the semiconductor layer 7 and the metallic electrode 8 is disposed directly on the fiber substrate.
However, in case of disposing the photodiode array directly on the fiber substrate, there are disadvantages as stated below. As is well known, the optical fiber has a structure wherein, as depicted in FIG. 4, a portion of high refractive index 9 called "core" is enveloped in a layer of low refractive index 10 termed "clad." The diameter of the optical fiber to be used is 50 .mu.m or less. The bundle of optical fibers 4 shown in FIG. 3 is fabricated by putting the optical fibers together and then fusing the clad portions by heat treatment so as to weld them together. The resultant bundle of optical fibers is shown in plan in FIG. 5. In this figure, numeral 11 indicates the clad portions joined together by fusion welding. The fiber substrate previously described is obtained by sandwiching the above bundle of optical fibers between members of the glass plate 5 and then bonding or fusion-welding the fiber bundle to the glass plate members.
The fiber substrate is formed of the three different materials. For this reason, in case where the surface of the fiber substrate is optically polished, it does not become smooth and inevitably involves some extent of unevenness. This condition will be explained with reference to FIG. 6. FIG. 6 is an expanded view of that part of a fiber substrate in FIG. 7 which is enclosed with mark . It is seen from FIG. 6 that a step a appears in the joined area between the glass plate 5 and the bundle of glass fibers 4 and that an unevenness b appears in the fiber bundle. a is approximately 150 nm, and b is approximately 50 nm. The step a and the unevenness b arise even when the optical polishing is carried out with scrupulous care. A concave part in the uneven portion corresponds to the clad, and a convex part to the core. When the photodiode array is disposed directly on such a fiber substrate, drawbacks as stated below are brought about.
FIG. 8 illustrates the portion of the photodiodes on an enlarged scale. The illustrated photo-sensor has the same structure as shown in FIG. 3. As apparent from FIG. 8, the step and unevenness in the fiber substrate cause concave and convex parts in the transparent conductor 6, the semiconductor layer 7 and the metallic electrode 8 which are provided on the fiber substrate. Such concave and convex parts are prone to give rise to the disconnection of the stripe electrodes 6 as well as the metallic electrode 8. In addition, they become a cause for the centralization of electric fields, to make the breakdown voltage of the photodiodes low and to make the dark current high. In this manner, the defect of the smoothness of the surface of the fiber substrate lowers the reliability of the photodiode array drastically and makes the yield of fabrication inferior.
Besides such disadvantages concerning the shape, the fiber substrate has disadvantages as listed below.
(1) Usually, the fiber substrate is washed before forming the photodiode array. In this regard, the fiber substrate is apt to be corroded by a solution of the hydrofluoric acid system which is generally used as a detergent. By way of example, merely by immersing the fiber substrate for 60 seconds in a buffer etchant in which hydrofluoric acid and ammonium fluoride are mixed at a volumetric ratio of 1 to 6, the unevenness (b in FIG. 6) becomes as large as 10 .mu.m, and the fiber substrate cannot be used at all. Therefore, perfect washing is difficult.
(2) In case of forming the transparent conductive film such as SnO.sub.2 conductive film 6 on the fiber substrate, it is ordinarily necessary to heat the fiber substrate to about 450.degree. C. The fiber substrate, however, is inferior in the heat resisting property. For example, when it is heat-treated at 450.degree. C. for 30 minutes, the unevenness (b in FIG. 6) becomes about 200 nm. For this reason, a method of forming the SnO.sub.2 transparent conductive film at a temperature below 400.degree. C. is required. In general, however, the SnO.sub.2 conductive film formed below 400.degree. C. has such disadvantages as inferior conductivity and easy exfoliation. It is accordingly inconvenient for forming the photodiode array.
As described above, the provision of the photodiode array directly on the fiber substrate involves a high possibility that the characteristics of the photodiodes will be degraded. In particular, the available percentage of the photodiode array will be lowered.